A matrix addressed display is organized to have a plurality of rows and columns for addressing and supplying data to the display elements. For example, liquid crystal displays (LCDs) typically comprise many rows (e.g., greater than 600) and normally even more columns (e.g., greater than 900) of liquid crystal light valves. For color displays, "color pixel clusters" are sometimes used where each cluster can comprise as many as 3 or 4 subpixels (1 or 2 RED, 1 BLUE and 1 GREEN).
In an LCD the brightness of each pixel is electrically controllable as a function of a column voltage supplied to each individual top pixel plate and a common backplane electrode. Such column voltages are typically generated in parallel by many (e.g. 8-24) column driver integrated circuits (ICs). Along with the application of column voltages, row scanning is accomplished by shift registers situated at one or both ends of the display. A bit is shifted through the register to activate each row. The column pixels for that row are then activated by the column drivers.
Presently, column drivers for available displays have 3-bit to 6-bit capability which, respectively, provides for 8 and 64 gray brightness levels. The brightness levels are designated by steps having individual heights within the available voltage range. Voltage step heights generally have to be non-uniformly spaced to achieve uniform brightness steps (as judged by the human eye). As expected, lower numbers of gray levels result in a lower quality image. This nonlinearity is achieved through a process known as gamma correction.
Present commercial column driver ICs capable of handling 128 to 192 columns have many significant drawbacks. First, they are specified to have column matching inaccuracies on the order of 25 mv to 100 mv on an integrated circuit (IC) where studies have shown that the human eye can detect column brightness striations in an LCD as low as 8 mv.
In operation, the column drivers provide voltage values which charge an individual line of a display during a line time. Each line in the display is successively selected to receive pixel data values from the column drivers until an entire picture has been formed. Individual lines are selected using, for example, a walking-one shift register coupled to row select circuitry (not shown) of a display (not shown).
One display system is shown in U.S. Pat. No. 5,162,786 to Fukuda entitled DRIVING CIRCUIT OF A LIQUID CRYSTAL DISPLAY. The column driver circuit described in this patent includes a digital to analog converter on each column which charges a pixel capacitance using an analog signal generated by a two-stage DAC. The first stage selects a pair of adjacent reference voltages from a set of 16 reference voltages. This pair of reference voltages is applied to the second stage which employs pulse-width modulation produce a signal which, when integrated, represents an eight-bit value. In order to ensure that the pixel capacitor is adequately charged, this circuit performs the D/A conversion operation 10 times during the line interval. These ten cycles are performed in 1.6 .mu.s. Assuming a line time of 16 .mu.s, the capacitance is being driven only 10 percent of the line time.
This low percentage drive time is undesirable if the display device is to be used in an environment containing significant levels of RF noise. This noise tends to redistribute charge on the array, causing the image pixels to blur.
Power considerations may be one reason that the drive time for this circuit is so low. If the circuit were to drive the pixel during a larger portion of the line time, it may draw too much power, causing heat problems in the display device. Alternatively, it may be that the low drive-time is due to the pulse-width modulation, integration of this type of signal over more than the ten cycles specified may degrade the accuracy of the digital to analog conversion.
Another disadvantage of prior art LCD display devices is the retention of charge on the LCD elements from image to image. Some existing devices compensate for this retention by switching the voltage ranges used to switch the liquid crystal from a range that is greater than the reference potential that is applied to the LCD back-plane to a range that is less than the reference potential. Because this switching of voltage ranges produces different gray scales, it may be distracting if the image is switched on a frame-by-frame basis.